Thrust/weight ratio has always been the major target of engine manufacturers seeking to improve military jets. Increasing compression ratio, increasing turbine inlet temperatures, and improving efficiency have led to a continuous reduction in engine size.
Conventionally, the power takeoff needed for driving accessories of the jet, such as the fuel pump, the oil pump, and the electricity generator, comprises a radial shaft driven by the compressor shaft via angle takeoff means, and in turn driving the accessories which are situated in a housing disposed to one side of the engine. The power takeoff and the accessory housing have had difficulty in keeping up with improvements in engines, and they now represent a very large fraction of the total mass of an engine, particularly of a small engine, particularly when the housing also contains a starter and an alternator.
The size of the accessory housing increases the frontal area of the engine.
However the use of small engines on trainer aircraft and also on attack or observation drones or on cruise missiles requires engine manufacturers to improve the furtiveness of such engines and thus to decrease their frontal area.
The mass and the frontal area of engines can be reduced by integrating a starter/generator in the jet and by driving the accessories using electric motors powered by the generator.
Under such circumstances, the mechanical power takeoff and the accessory housing can be made smaller or can be omitted.
The use of electric motors for driving accessories, such as fuel pumps, lubricating oil pumps, and hydraulic pumps, presents the advantage of making the accessories easier to control and of enabling them to be placed anywhere in the airframe or the engine mounting pylon in locations that make them more accessible and easier to replace, while nevertheless reducing their vulnerability to possible enemy fire.
The only engine/accessory interface then becomes an electrical transmission. It constitutes an electricity network which conveys energy from the auxiliary power unit to the starter and to the electric motor that drives the fuel pump while starting the engine, after which, once the jet is capable of running on its own, it is the integral starter/generator which supplies electricity to the electric motors driving the accessories, in particular the motor driving the fuel pump.
However, in spite of the high degree of reliability of present-day electrical systems, it is always possible for an electrical breakdown to occur in flight, either on the electricity generator, or on the electric motor driving the fuel pump. That will cause the engine to go out and can lead to loss of the airplane or other aircraft if it has only one engine.